This invention relates to pneumatic tires for motorcycles, particularly high-speed running type motorcycles. More particularly, it relates to a pneumatic tire for motorcycle having improved high-speed durability.
In general, this type of the tire comprises a carcass of a so-called bias structure wherein at least two plies each containing organic fiber cords therein are crossed with each other at a cord angle of 26.degree.-36.degree. with respect to the equatorial plane of the tire, and a tread portion of rubber superimposed about a crown region of the carcass. The rubber thickness of the tread portion is substantially equal over the crown region, so that the surface of the tread portion has a substantially parallel relation to the contour of the carcass along such contour.
Recently, the tendency for high-speed running of vehicles has increased with the development of road networks, particularly with the opening of expressways accompanied with the perfection of paved roads. This tendency is also true in case of motorcycles. As a result, a requirement exists to improve the high-speed durability of the tire.
In case of high-speed running type motorcycles, there is a difference in the tire size between the front wheel and the rear wheel as compared with the case of four-wheeled passenger vehicles, so that the balance of cornering stability between the front and rear wheels is a significant subject.
The conventional tire for motorcycle as described above is apt to produce separation failure between the tread portion, particularly tread central region and the carcass under an influence of a strong centrifugal force when the tire is subjected to high-speed running, particularly a super high-speed running test for assuring ordinary performances of the tire, whereby it is obliged to considerably reduce the high-speed durability.
Heretofore, it has been attempted to apply the belt reinforcement, particularly steel cords usually used in tires for passenger cars to the tire for motorcycles in order to control the aforementioned centrifugal force. Contrary to expectations, however, high-speed durability is not so improved and inversely, cornering stability in the high-speed running, particularly cornering at high speed is deteriorated considerably.
The inventors have made various investigations and experiments in order to improve high-speed durability without sacrificing cornering stability in pneumatic tires for high-speed running type motorcycles and found that this object is effectively achieved by a tire reinforcing construction based on a new conception relating to a carcass of a bias structure and an arrangement of a breaker surrounding the carcass. As a result the invention has been accomplished.
According to the invention, there is provided a pneumatic tire for motorcycles comprising a carcass composed of at least two rubberized plies each containing organic fiber cords therein, the cords of which being crossed with each other at an inclination angle with respect to the equatorial plane of the tire, a tread portion of rubber extending over the crown region of said carcass along the contour thereof at a substantially equal thickness, a breaker disposed between said carcass and tread and composed of at least one rubberized ply containing organic fiber cords therein, and a side portion of rubber covering both sides of said carcass, the improvement wherein said carcass and breaker are arranged in substantially parallel with each other, said carcass has a cord angle of 26.degree.-36.degree. with respect to the equatorial plane, and said breaker has a cord angle of 0.degree.-25.degree. with respect to the equatorial plane and extends at a width corresponding to 30-200% of the width of said tread portion.
In the preferred embodiment of the invention, the breaker has a width corresponding to 50-80% of the tread width, the breaker is composed of two cord plies crossed with each other, the cords constituting the breaker have substantially the same properties as that of the carcass, the cord angle of the breaker is within a range of 10.degree.-20.degree. with respect to the equatorial plane, the coating rubber for the breaker is the same as that of the carcass, the breaker is substantially arranged in direct contact with the carcass, and the tread portion extends over the actual maximum width of the tire.
According to the invention, it is necessary that the tread portion has the substantially equal thickness extending over the crown region of the carcass along the contour thereof. This is due to the following fact; that is, when the body of the motorcycle is inclined toward the cornering side during the running, a so-called camber angle or an inclination angle of the equatorial plane of the tire with respect to a plane perpendicular to horizontal plane is given to the tire, whereby a camber thrust is produced to the tire. In order to always and smoothly produce camber thrusts under various camber angles, it is necessary that a distance from the carcass contour to the surface of the tread portion is really constant over the actual crown region of the carcass. For this reason, it is also necessary that the breaker disposed on the carcass and developing the composite reinforcing effect with the carcass is arranged in substantially parallel with the carcass.
The cord angle of the carcass should be within a range of 26.degree.-36.degree., which is somewhat smaller than the conventionally used cord angle, in view of the harmony with the cord angle of the breaker of 0.degree.-25.degree., preferably 10.degree.-20.degree.. When the cord angle of the breaker is larger than 25.degree., a so-called hoop effect to the carcass is reduced, while the bending rigidity in the radial plane of the tire increases, so that it is disadvantageous in the high-speed durability and cornering stability.
In the arrangement of the breaker, the width of the breaker should be 30-200%, preferably not less than 50% to the width of the tread portion. When the breaker width is smaller than 30%, the extension of the carcass in radial direction cannot be sufficiently suppressed. Moreover, the breaker width of not less than 80% contributes to increase the rigidity of the side portion of the tire, so that the widening of the breaker up to 200% advantageously produces a composite reinforcing effect with the carcass.